There was a debate about this a few years ago. I had no strong opinion at the time. But, with hindsight, I now support 100% Luis Argerich's decision. I think it's more work & risk to integrate an external forum app (not to mention licenses, community aspects, etc) that to develop our own.
The only advantage would have been to leverage exisiting communities and get more users. We could achieve these goals with less effort, via a data migration script and a phpBB look alike theme like this one:
Even Bitweaver is now offering BoardsPackage to replace phpBB.
While phpBB is nice & popular, there are some features, like file attachments to a post which have been in Tiki for years and are not yet in the stable version of phpBB (will be in 3.0)
OTOH, The user interface of Tiki forums was not so great. It has been improved in soon to be released 1.9.8 and even more in 1.10 (in dev)
AFAIK, phpbb import is still on wishlist: